1. Field of the Invention
This invention pertains generally to particle accelerators, and more particularly to dielectric wall accelerators.
2. Description of Related Art
In a conventional induction accelerator, the beam pipe is conducting, so that an accelerating electric field is present only in the gaps between accelerator stages. Thus the accelerating field occupies only a relatively small fraction of the axial length of an accelerator cell.
In a dielectric wall accelerator (DWA), an insulating wall replaces the conducting beam pipe. The dielectric wall is energized by a pulsed power system. The accelerating fields can then be applied uniformly over the entire length of the accelerator, yielding a much higher gradient, e.g. 20 MeV/m or more, compared to about 0.75 MeV/m. A high gradient DWA can thus be made much more compact than a comparable conventional induction accelerator.
A number of technological developments have led to DWA designs with greatly enhanced performance. An insulator material, called a “high gradient insulator” (HGI), made of alternating layers of conductor and insulator with periods on the order of a mm or less, has a much higher surface flashover threshold than monolithic insulators. Solid dielectrics have high bulk breakdown strength and can be used in high voltage pulse generators. Photoconductive switches using wide band gap materials such as SiC or GaN are compatible with very high voltage gradients and are advantageous to initiate the output voltage pulse in a DWA.
An important part of the DWA is the pulse forming system. A wide variety of pulse generating lines employing closing switches are generically referred to as “Blumleins.” These lines are made up of two or more transmission lines, either planar strip lines or radial lines. The Blumlein is actuated to generate a pulse by closing a switch, typically a photoconductive switch. In a typical DWA configuration, two stacks of strip Blumleins are placed on opposite sides of the beam tube.
To efficiently accelerate charged particles axially along the beam tube, the particles should always be embedded in an accelerating field. To do so, the region of the dielectric wall exposed to a high electric field must move along with the accelerating particles. This can be done by making the Blumleins relatively thin and activating them in sequence to produce a region of excitation along the wall that maintains synchronism with the charged particles. Thus, as the electric field produced by the pulse generating Blumleins propagates down the bore of the accelerator, it pushes the packet of charged particles before it.
Although it has higher impedance and requires fewer switches than a radial line, the strip Blumlein suffers from parasitic coupling between different lines in a stack. This coupling occurs because electric and magnetic fields leak axially from layer to layer. This leakage causes temporal distortion of the pulse and a reduction in amplitude. Thus, the accelerating gradient is reduced from its theoretical ideal value.
Other problems with Blumlein actuated DWAs include the large number of switches required for the accelerator, about one switch per mm; the relatively large energy required to achieve high gradient, and the total laser energy required for the accelerator. During charging of the lines, the Blumlein switches are in the off state, and are subject to large voltage gradients for long periods of time, typically hundreds of nanoseconds or longer, producing high electrical stress on the switches. The Blumleins output into an open circuit to attain maximum gradient, leading to ringing of the lines and voltage reversals on the dielectric wall. There is also strong radial defocusing on the particle beam, and there is no room to add external focusing.
One area where a compact high gradient accelerator would be of great advantage is a proton accelerator for medical applications. The benefits of proton therapy over x-ray therapy are well known. However, at present proton beams are produced in very large accelerators, and very few medical facilities have such a machine. A compact proton accelerator that could replace x-ray machines would greatly expand the availability of proton treatment.